• SkillCat Team

Refrigeration Equipments: Evaporator Properties & Types

Updated: Jun 30

Refrigeration Equipment : Chapter 1


Evaporator


In this module, we will go over the evaporator component in more detail. We will also talk about the types of evaporators used across different appliances. Skip to quiz!


Overview


Recall that the purpose of the evaporator is to absorb heat from the air. Absorbing heat from the air is what provides cooling in the refrigeration cycle.


Evaporators contain a series of coils filled with refrigerant. These coils are the main part of the evaporator. Remember that refrigerants absorb heat by using the air’s heat to boil, changing phase into a vapor.

The evaporator coils increase the total surface area over which air can flow. This speeds up the heat removal process.Increasing surface area will always speed up the rate of both heating and cooling.


To think about how surface area affects heat transfer, let’s think about making a steak. When we cook a steak, we cook it flat side down. The part of the steak that is in contact with the pan is the surface area where heat transfers to the steak and cooks it.


Now, let’s think for a second about cooking a steak as is pictured here. Our gut tells us that this is wrong. We know that the steak would not cook evenly.


In this example, the surface area where heat can transfer from the pan to the steak is much smaller. So it would take much longer to cook the whole steak, and the steak would cook unevenly. We see with this example that the larger the surface area, the faster heating can happen. The same is true for cooling.


In the evaporator coils, the refrigerant starts as a liquid. It absorbs heat from indoor air to change state to a vapor. In this picture, we see the solid blue color, which is the liquid state of the refrigerant. The bubbles indicate the vapor state the refrigerant turns to.


So in the evaporator, we have refrigerant in both

  • Liquid phase, and

  • Vapor phase.

It is important to remember what state the refrigerant is in for each component of the refrigeration cycle.


The specific requirements for an evaporator depend on the type of system we are looking at. There are three main categories of systems:

  • Refrigeration Systems,

  • Air Conditioning, and

  • Chillers.

We will take a look at the types of evaporators used for each type of system.


Refrigeration Systems


Refrigeration systems are appliances that keep objects cold. They can either be:

  • Residential Refrigeration Systems, or

  • Commercial Refrigeration Systems.

Residential refrigeration systems are cooling appliances seen in your typical home. Examples of residential refrigeration appliances include your

  • Fridge

  • Freezer, or

  • Dehumidifier

Commercial refrigeration systems are appliances that cool things on a larger scale. Examples of commercial refrigeration appliances include your

  • Supermarket refrigerators,

  • Refrigerated warehouses, and

  • Restaurant refrigeration systems

In refrigeration systems, the evaporators used are:

  • Compact

  • Cost efficient, and

  • Easy to clean.

With refrigeration systems, we’re talking about cooling a volume or space that is constant and relatively small. For example, the volume of your average fridge is about 20 cubic feet. So the space you need to keep cool is about 20 cubic feet.

Because refrigeration systems are relatively compact, the size of their evaporator components would also be more compact. There are three types of evaporators most commonly used in residential and commercial refrigeration.


These three evaporator types are:

  1. Bare Tube Evaporators

  2. Plate Surface Evaporators, and

  3. Finned Tube Evaporators.

We will go into more detail about how each of these evaporators work in following modules.


The three types of evaporators are used in both residential and commercial refrigeration systems. But because commercial systems are larger than residential systems, the size of their evaporators would be larger.


Air Conditioning Systems


Air conditioning systems are systems that provide comfort cooling. Examples of air conditioning systems are:

  • Window AC units,

  • Split System AC units, and

  • Rooftop units.

AC units can either be residential or commercial.


Since the goal of air conditioning systems is to cool air, evaporators used in these systems have to take airflow into account.

In air conditioning systems, new air will constantly be pushed through the evaporator so that the refrigerant inside can remove heat from the air. In AC systems, we need to move air through the system, as opposed to refrigeration systems. In refrigeration systems, we are not concerned about airflow because air generally doesn’t move much in a fridge.


In air conditioning systems, we primarily use finned tube evaporators. This is because finned tube evaporators have the most surface area out of the three types of evaporators that we’ve talked about so far.


The finned tube evaporators we see in air conditioning systems can have a variety of shapes. The four evaporators we see in this graphic are all called finned tube evaporators. We will take a look at each of these coils and how air flows through them differently in a later module.


Chillers


A chiller is a piece of equipment used to produce cold water. Chillers are generally large pieces of equipment that are used in industrial settings such as:

  • Cooling machinery,

  • Injection molding, and

  • Cooling large buildings.

Evaporators used in chillers are generally shell and tube evaporators, as shown in this video. A shell and tube evaporator consists of a copper tube bundle inside a large outer shell.


The copper tubes contain the water that we are trying to chill. In this video, the red represents the hot water flowing through the inner copper tubes in the evaporator.


The outer shell of the evaporator is a large container of refrigerant fluid. This shell is sealed off and leakproof to prevent refrigerant from escaping. Heat will transfer from the water to the outer shell that contains refrigerant, which will cool the water.


Shell and tube evaporators are generally very efficient at cooling large quantities of water. This is why shell and tube evaporators are used for chillers in industrial or commercial settings where large quantities of cold water are needed.


In this module, we discussed the different types of evaporators and compared what they are used for. Different types of evaporators are used for different systems because the requirements for each system are not the same. Next, we will take a more detailed look at properties of the different evaporators.





Evaporator Parts


In this module, we will go over the main parts of an evaporator and how they work. Skip to quiz!


Overview


The evaporator’s job is to absorb heat from a space in order to cool the space. It does this by passing refrigerant through a series of coils, where they boil using heat from the air.


The evaporator component can be broken down into a few key parts:

  • Refrigerant Tubes,

  • Inlet and Outlet Pipe,

  • Fins, and

  • Drain Pan

In the rest of this topic, we will go over the function of each of these parts.



Refrigerant Tubes

Refrigerant tubes are designed to hold refrigerant and to move refrigerant around a system.


These refrigerant tubes are usually made of metals like copper or aluminum because they are good conductors of heat.These metals allow heat to freely flow from the air to the refrigerant. The easier it is for heat to transfer to the refrigerant, the more efficient our evaporator is at cooling.


Evaporator tubes increase the surface area over which air can flow. This speeds up the cooling process, as we talked about in the previous module.


Inlet and Outlet Pipes

The inlet and outlet pipes carry refrigerant into and out of the evaporator. Like refrigerant tubes, they are made of metals like copper or aluminum.


Remember that in the refrigeration cycle, the metering device comes before the evaporator. And the compressor comes after the evaporator.


So in the cooling cycle, the inlet pipe would bring in refrigerant that just left the metering device. This refrigerant coming into the evaporator would be liquid.


The outlet pipe connects to the compressor, which is the next component in the refrigeration cycle. The outlet pipe would carry refrigerant that is fully vapor because the refrigerant would have boiled in the evaporator.


Fins


Fins are very thin circular or rectangular plates that are come attached to refrigerant tubes. Fins are generally made of aluminum because it is a good conductor of heat.


Remember we said that both copper and aluminum are good conductors of heat. But fins are made of aluminum, not copper. This is because copper is soft and bendable, which makes it easier to damage than aluminum.

The purpose of fins is to increase the surface area over which air can flow. As as we demonstrated, the larger the surface area, the faster and more efficient heat can transfer from the air to the refrigerant inside the tubes.


To see how fins can increase surface area, let’s take a look at this single tube. Let’s say the length of this tube is 5 inches. The highlighted area is the surface area over which air can flow.


Now, let’s add 4 fins to to this tube. When air flows over the tube, it also flows over the fins and the tube. Since the fins are made of aluminum, which is a good heat conductor, any heat that is transferred to the fins will transfer to the tube. Now, the entire highlighted area is the surface area over which air flows. We can see that this area is larger than just the surface area of the tube.


The more fins we add to the tube, the more the surface area increases, and the more heat can be transferred to the refrigerant inside the tube. Of course, we can’t just stack the fins all together because then, there would be no airflow between the fins. So the design of fins is a balance between the number of fins and how much air as to pass through in between the fins.


Not every evaporator will have fins. The evaporator on the left is called a bare tube evaporator. It does not have fins. The evaporator on the right is a finned tube evaporator. As we can tell from its name, it does have fins.


Drain Pans


Remember that condensation happens when a vapor turns into a liquid because of a decreasing temperature. When hot air flows over refrigerant tubes containing cold refrigerant, this will produce condensation. Let’s take a closer look at how this happens.


Let’s think about a single section of a simple refrigerant tube in a warm room.The room is a higher temperature than the refrigerant inside the tube. That means all the vapor or gas molecules in the room’s air are also at a higher temperature than the refrigerant tube.


If you touch the refrigerant tube, you know that it will feel cold because the tube holds refrigerant at a low temperature.


Air naturally contains a percentage of water vapor molecules. When these water vapor molecules in the air bump against the cold refrigerant tube, the water vapor molecules will also feel cold.


We know that heat will always transfer from high to low. When water vapor bumps against the cold refrigerant tubes, heat will transfer from the water vapor to the cold tube. When the water vapor loses heat from this heat transfer, it will condense into a liquid.


So as an evaporator operates and refrigerant flows through it, water will condense on the refrigerant tubes and drip off. A drain pan collects this condensation to prevent damage to our systems because metal can rust. Drain pans connect to a pipe that drains this condensation.


In this module, we discussed the parts of an evaporator, including refrigerant tubes, inlet and outlet pipes, fins, and drain pans. Next, we will talk more about how an evaporator works.






Evaporator Properties


In this module, we will go over properties that affect how well an evaporator works. These properties include surface area, airflow patterns, and frosting. Skip to quiz!


Surface Area


Recall that surface area is the total area of all the faces of a three dimensional object.


We can imagine surface area as the minimum amount of wrapping paper we would need to cover an object. For example, for this box, we can visualize its surface area as the total area of all its surfaces.


For our purposes, we are interested in the surface area of evaporator coils. The evaporator’s job is to absorb heat from the air as fast as possible to provide cooling. And as we covered in the previous module, the larger the surface area of the evaporator, the faster heat can transfer.


We can increase surface area by:

  • Increasing the length of evaporator coil,

  • Adding fins, or

  • Both

Increasing the length of the coil in the evaporator will increase the surface area. For example, let’s say we start with this tube. The surface area of this tube is highlighted. At every point of this highlighted area, heat exchange happens where the heat from the air will transfer into the refrigerant tube.


Now, lets double the length of this tube. This will double the surface area over which air can flow. With this longer tube, heat will transfer faster.


We can also increase the coil length by adding another layer of coils. We can see with this evaporator design that there are stacks of evaporator coils.The more stacks of coils there are, the larger the overall surface area, and the faster heat will transfer. Of course, we have to make sure that the coils are not stacked so close together that air cannot flow through.


Adding fins to refrigerant tubes will also increase the surface area that air can flow through. Air can now also flow over the additional area of the fins. Since the fins are metal, they will conduct this heat through to the refrigerant.



Airflow


In refrigeration and air conditioning systems, there is generally a blower fan whose job is to blow air onto the evaporator coils. The blower fan’s job is to direct warm air to flow through the evaporator coils so that the air can be cooled.


The blower fan can be on top of the evaporator or below, or on the same level. The position of the blower fan is different depending on the space it is installed in and the type of system.


Where the blower fan is placed will affect how air flows through the evaporator. There are four main types of airflow, categorized by the position of the evaporator relative to the blower fan:

  • Upflow

  • Downflow

  • Horizontal, and

  • Multiposition


An upflow of air is when the evaporator is on top of the blower fan. In residential applications, upflow configurations are installed in the basement. Air is pushed upwards, through the evaporator.


Downflow is when the evaporator coils are below the blower fan. This type of installation is seen in attics because air naturally settle downwards.


In horizontal airflow, the blower fan and evaporator coils are in line with each other. This installation is used when space is limited.


Multiposition is exactly as it sounds. It’s a combination of all the type of flow we just discussed. Multiposition systems can be adjusted to blow air through the evaporator in any direction, as needed.



Defrosting


Recall that there is may be condensation outside the evaporator’s refrigerant tubes.


This happens because the refrigerant coils are cold and the surrounding air is hot. Water vapor naturally exists in the air. When these water vapor molecules bump into the cold evaporator coils, heat transfers from the “hot” water vapor molecules to the cold refrigerant coils.

Because the water vapor molecule’s temperature decreases, it changes state to a liquid water molecule. This is why water drops form on the surface of evaporator coils. It’s similar to the water drops that form on the outside of a glass of ice water. And this is also why we have drain pans.


If too much condensation collects on the surface of the evaporator coils, this can prevent air from coming into contact with the coils. And since the evaporator coils are cold, mostly below freezing, the water formed on the outside of the coils can freeze. This is why a thick layer of frost can form over the coils.


We do not want frost over the evaporator coils because this means that hot air can’t come into contact with the coils. The frost decreases the available surface area and creates cooling inefficiency. If there is frost, the evaporator cannot cool properly.


Frost can also form if the air is too humid or refrigerant is leaking. If the air is too humid, there are more water vapor molecules in the air. This means more of the water vapor molecules will condense onto evaporator coils.


This is why we need to remove the frost if we see it forming. This process is called defrosting. To defrost, we can use one of three main methods:

  • Air Defrost,

  • Electric Defrost, or

  • Hot Gas Defrost


Air defrosting involves shutting down the system and letting warm air pass over the evaporator coils. Air will melt the ice formed over the evaporator coils. The air doesn’t have to be hot, even room temperature will do. Recall that if we leave an ice cube out in room temperature, it will melt. This is the same idea.


Some appliances are equipped with defrost heaters that are powered by electricity. These heaters are built into the coils and will warm up the coils locally to melt the built up ice. These electric defrost heaters can be activated manually or automatically if a sensor detects ice buildup.


In a hot gas defrost system, we use the heat from the compressor to defrost ice buildup. We know that the refrigerant in the compressor is a high temperature vapor that is then sent to the condenser to release the heat. In hot gas defrost systems, this hot vapor is to the evaporator to melt the frost instead.


Hot gas defrost system is faster than electric defrost systems. However, it can only be seen in large systems that have multiple evaporator coils. Using the hot gas defrost in smaller systems does not make sense economically.


In this module, we discussed how to think about the surface area of evaporator coils, and the types of airflow. We also talked about why frost forms on the evaporator and how to defrost.





Evaporator Types


In this module, we will go over different types of evaporators and how each of them work. Skip to quiz!


Overview

The evaporator’s job is to absorb heat, which provides cooling. The type of evaporator depends on what type of appliance it is used in. This makes sense, since the requirements for a refrigeration system would be different than the requirements for an air conditioning system.


All evaporators are made of metals like copper and aluminum because they are good conductors of heat. A good conductor allows heat to transfer freely from air outside the coils to the refrigerant inside the coils.


There are four main types of evaporators.

  • Bare Tube Evaporators,

  • Plate Surface Evaporators,

  • Finned Tube Evaporators, and

  • Shell and Tube Evaporators.

We will take a look at how each of these work.


Bare Tube Evaporator


Bare tube evaporators are the simplest type of evaporator. These evaporators are generally just refrigerant tubes that are made of either copper or aluminum.


There is refrigerant inside the refrigerant tube that absorbs heat from the air and vaporizes inside the tube. Heat transfers across the surface area of the metal refrigerant tubes.


We generally find these evaporators in household refrigerators and freezers. This is because bare tube evaporators are less effective than the other evaporators we will talk about. These evaporators will not work for large cooling requirements in commercial uses.


Bare tube evaporators do have some advantages. They are:

  • Easy to clean, and

  • Easy to defrost.

This makes them suitable for household refrigerators and freezers.


Plate Surface Evaporator


Plate surface evaporators are flat, as shown in this image. These evaporators are made up of two plates that are joined together. Usually the two plates are made of aluminum because it conducts heat well.

Liquid refrigerant flows through a tube made between the two plates. This tube carrying refrigerant is the raised part of the plate that we can see in this image.


The refrigerant flows through the tube and absorbs heat from the air. Because the refrigerant tube touches the plates, the aluminum plates act to increase the surface area that heat can transfer. Heat can transfer to the aluminum plates, and because the plates are connect to the tubes, heat transfers from the plate to the refrigerant in the tube.


Plate surface evaporators are an improvement over bare tube evaporators because they have a higher surface area for air to flow through. The higher the surface area, the more heat can be transferred into the refrigerant.


Plate surface evaporators are generally used in small appliances like household refrigerators, home freezers, beverage coolers, or ice cream cabinets.In plate surface evaporators, food can be placed directly on the plate for more effective cooling.


The benefits of plate surface evaporators are that they are:

  • Easy to clean and defrost,

  • Less likely to leak refrigerant, since the plates protect the tube carrying refrigerant, and

  • Safe to handle.

Finned Tube Evaporator

The finned tube evaporator is more complex and more effective. A fin is a very small plate that is attached to the body of the evaporator. As you can see in this image, finned tube evaporators have a lot of fins attached to its body.


At each fin, there is a transfer of heat into the evaporator coils. The more fins there are, the more heat that can be transferred This is what makes the finned tube evaporators the most effective of them all.


Since the fins are made of metal and can be sharp, we have to be careful when handling finned tube evaporators. During service, we also need to be careful because we can damage the fins since they stick out.


Finned tube evaporators can have four different shapes. The shape they have depends on the space we are designing for. There are:

  • Slab Coils,

  • Slant Coils,

  • A Coils, and

  • N or M Coils.

Slab coils are shaped exactly like they sound. They do not contain as much surface area as other types of coils, so they are limited in use. Slab coils are generally used in small spaces because they can fit more easily than the other types of coils.


Slant coils are shaped almost exactly like slab coils. The difference is that they are placed at a diagonal, which allows them to be longer than either the length or width of the space. This means allows for a higher cooling efficiency because there is a higher surface area for air to flow through.


A coils are the most common type of coil seen in the industry. They are made of two slant coils, which almost doubles the surface area that air can flow through. This means that A coils are almost twice as effective as slant coils!


N or M coils are sometimes called Z coils because of their orientation. They are shaped as they sound. It is believed that the third side to the coils increases efficiency because there is more surface area for the air to flow through. But these coils are subject to freeze ups and can be hard to clean.


Shell and Tube Evaporator


A shell and tube evaporator consists of a copper tube bundle inside a large outer shell. They are generally very efficient at cooling large quantities of water. This is why shell and tube evaporators are used for chillers in industrial or commercial settings where large quantities of cold water are needed.


The copper tubes contain the water that we are trying to chill. In this video, the red represents the hot water flowing through the inner copper tubes in the evaporator.


The outer shell of the evaporator is a large container of refrigerant fluid. This shell is sealed off and leakproof to prevent refrigerant from escaping. Heat will transfer from the water to the outer shell that contains refrigerant, which will cool the water.


In this module, we discussed the different types of evaporators and compared what they are used for.





Question #1: The larger the total surface area of the evaporator coils, the faster air can be cooled.

  1. True

  2. False

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Answer: True

This is true. If you have a larger surface area, there are more points of contact, which will make the heat transfer faster into the evaporator coils.



Question #2: In the evaporator, what state is refrigerant in?

  1. Liquid

  2. Vapor

  3. Both liquid and vapor

  4. Solid, liquid, and vapor

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Answer: Both liquid and vapor

In the evaporator, the refrigerant starts as liquid. As it boils, it turns into vapor. So in the evaporator, there is both liquid and vapor refrigerant.



Question #3: Evaporators used in refrigeration systems are:

  1. Relatively cheap

  2. Relatively small

  3. Easy to clean

  4. All of these

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Answer: All of these

Evaporators used in refrigeration systems are:

  • Cost effective, or relatively cheap

  • Compact, or relatively small in size

  • Easy to clean


Question #4: Evaporators used in commercial refrigeration systems are the same type and size as residential refrigeration systems.

  1. False

  2. True

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Answer: False

This is incorrect. Commercial refrigeration systems use the same type of evaporator coil as residential systems. But the size of the coil in commercial systems would be larger.


Question #5: Evaporators in air conditioning systems have to take into account ____ air.

  1. Moving

  2. Still

  3. Weird

  4. All of these

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Answer: Moving

The air in air conditioning has to circulate through the space of a house or building. So the evaporator has to account for air that is moving.


Question #6: Air conditioning systems use what type of evaporators?

  1. Bare tube evaporators

  2. Plate surface evaporators

  3. Finned tube evaporators

  4. All of these

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Answer: Finned tube evaporators

Air conditioning systems use finned tube evaporators.


Question #7: Finned tube evaporators should be used for chillers because they have high surface area.

  1. True

  2. False

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Answer : False

This is false. The only type of evaporator used in chillers is shell and tube evaporators.


Question #8: Shell and tube evaporators are used:

  1. In chillers

  2. For cooling large quantities of water

  3. Both of these

  4. None of these

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Answer: Both of these

Shell and tube evaporators are used in chillers. Generally, chillers are used in industrial settings for cooling large quantities of water.


Question #9: Refrigerant tubes should be made of insulators because they increase efficiency.

  1. True

  2. False

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Answer: False

This is not true. Refrigerant tubes are made of heat conductors because we want heat to transfer as easily as possible.


Question #10: Refrigerant tubes ____ the surface area over which air can flow.

  1. Increases

  2. Decreases

  3. Has no effect on

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Answer: Increases

Refrigerant tubes increase the surface area over which air can flow.


Question #11: Inlet and outlet pipes are used to transport refrigerant into and out of the evaporator.

  1. True

  2. False

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Answer: True

This is true. The inlet pipe would bring in refrigerant that just left the metering device. The outlet pipe would carry refrigerant that is fully vapor because the refrigerant would have boiled in the evaporator.


Question #12: Fins____________________.

  1. Are made of aluminum

  2. Are of copper

  3. Are made of both aluminum and copper

  4. Are made of all types of metals

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Answer: Are made of aluminum

Fins are made of aluminum because it is harder than copper. This is important to prevent damage to the fins.


Question #13: Fins_________________.

  1. Are made of silicone

  2. Are made of metal

  3. Are on all refrigerant tubes

  4. All of these

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Answer: Are made of metal

Fins are made of metal because it is important for them to transfer heat to the refrigerant inside the refrigerant tubes. Not all refrigerant tubes have fins.


Question #14: Finned tube evaporators are generally more efficient than evaporators without fins. This is because the fins increase the surface area over which air can flow.

  1. True

  2. False

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Answer: True

This is true. The more fins we add to the tube, the more the surface area increases, and the more heat can be transferred to the refrigerant inside the tube.


Question #15: Increasing the number of fins

  1. Increases the surface area that air can flow over

  2. Can be ineffective in some cases

  3. Both of these

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Answer: Both of these

Both of these are true. Increasing the number of fins increases the surface area over which air can flow. But as we add more and more fins, at a certain point, not enough air can flow over the fins. Too many fins would be ineffective.


Question #16: In this scenario, if you touch the refrigerant tube, will it be hot or cold?

  1. Hot

  2. Cold

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Answer: Cold

It will be cold because the refrigerant is at a lower temperature than room temperature.


Question #17: Condensation happens when water vapor molecules inside refrigerant tubes move through the tube and drain out of the evaporator.

  1. True

  2. False

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Answer: False

This is false. Condensation happens when water molecules outside the refrigerant tubes touch the cold refrigerant tubes. Heat transfers from the hot air to the cold tubes, so heat transfers away from the water vapor molecules. Decreasing temperature condenses these water vapor molecules.